1) Field of the Invention
The present invention relates to an image formation apparatus, an image formation method, and a program for allowing a computer to execute the method.
2) Description of the Related Art
Recently, there are many image formation apparatuses that form a color image, such as color copiers and color printers. Such a color image formation apparatus (for example, a color printer) often includes a powdered ink (toner) of a plurality of colors, for example, yellow (Y), magenta (M), cyan (C), and black (K). With such a color printer, a color image is digitalized, and the digitalized image data is input to the printer, and the color image is formed (printed) on a sheet of paper by each color toner or toners of a plurality of colors that are mixed, based on the image data.
Some of the color printers prints, for example, black or gray only with the K toner being the toner color. Printing only with one color toner can prevent occurrence of toner scattering or color unevenness caused by superposing the toners, and therefore an image having a neat impression can be formed. Further, in a color printer that forms images in which color continuity is valued, like a color picture, gray is often expressed by mixing each toner of C, M, Y, and K.
Color mixture by the color printer is carried out using a color conversion table. Most of the color conversion tables are a three-dimensional look-up table (LUT) for color conversion. The color conversion table is used for converting an input color determined by three elements, red (R), green (G), and blue (B), into an output color determined by C, M, Y, and K. Representative input colors (model colors) are, as shown in FIG. 11, discrete values indicated by points of intersection (black spots) of solid lines intersecting each axis of R, G, and B, and an output color corresponding to an input color (white spot) located between the model colors can be obtained by interpolation processing using some of the model colors. When the output color is determined by interpolating the model colors, the output color is adjusted taking the continuity with the model colors into consideration.
However, the output color obtained by the conversion generally changes due to a change in properties with time of the color printer and an individual difference between the color printers. Such a change occurs, for example, when a color image formed by a color printer at the time of shipment has a different hue from that of the 1000th color image formed based on the same image data, or when the hues of color images obtained by printing the same color image by a plurality of color printers are different from each other.
When the output color corresponding to each model color in the color conversion table changes, the output color obtained by interpolation using the model colors also changes. Therefore, the tone continuity between the output color obtained through interpolation and the output color corresponding to the model colors is disrupted. The disruption of the tone continuity is easily recognized by human, particularly in the gradation from a toner color to a color having the same hue (for example, from a toner color to white, or from a toner color to black), and may deteriorate the quality of the color image.